Nitrogen oxide emissions from rich premixed reacting jets in a vitiated crossflow

Abstract

This paper describes NOx measurements from reacting jets in crossflow (RJICF). This work is motivated by interest in axial staging of combustion as a means of reducing NOx emissions at high flame temperatures (>1900 K), where thermal NOx production rates are high. In this approach, the majority of the fuel is burned in a conventional lean-premixed flame, but additional fuel is injected from the combustor walls into the vitiated flow further downstream. The NOx emissions of RJICF are influenced by the secondary fuel jet stoichiometry, jet/crossflow mixing before combustion, as well as secondary combustion product mixing with the bulk product stream. In turn, jet/crossflow mixing is controlled by the hydrodynamic stability of the jet, as well as degree of flame lifting. A key challenge in understanding fundamental factors influencing NOx is decoupling the effect of bulk temperature rise due to the RJICF (∆T), JICF momentum flux ratio (J), and JICF stoichiometry (ϕJet), as they cannot be varied independently. As such, significant effort was made in developing a test matrix to differentiate their effects. Measurements reported here were obtained from rich premixed methane/air jets injected into a varying temperature (1650 K–1800 K) vitiated crossflow, for bulk temperature rises from 20 K–290 K, J values from 1.3–4.4, and ϕJet values from 1–9. These measurements show that NOx emissions monotonically increase with ∆T, as noted in prior studies, but the data reported here are able to differentiate the effects of ∆T and other parameters. In fact, for a given ΔT value, NOx values can vary by 2X depending upon other parameters. For example, the lifting of the flame (LO), which varies with ϕJet and J has significant effects on NOx emissions. These data suggest that the key fundamental JICF parameters influencing NOx emissions are ∆T, ϕJet, J, and LO.